No abstract

In rocky intertidal habitats, the pronounced increase in environmental stress with elevation, caused by tides, generates significant vertical variation in community structure. Along coastlines, environmental changes generate horizontal biological variation, which, when measured at large sampling intervals, generally surpasses vertical biological variation. It is unknown, however, how vertical biological variation compares with horizontal biological variation when the latter is measured in environmentally similar habitats. We tested the hypothesis that, for sites experiencing similar environmental conditions along the shore, horizontal biological variation does not surpass vertical biological variation even when horizontal variation is measured at large sampling intervals along the coast. We compared vertical and horizontal variation in intertidal communities by surveying habitats experiencing the same wave exposure on the NW Atlantic and SE Pacific coasts. We measured biological variation based on differences in species richness, occurrence, and abundance between quadrats from low and high elevations (vertical variation) and between quadrats at three horizontal scales of sampling interval on both coasts: local (tens of cm between quadrats), meso-(~100 km between quadrats), and regional (~200 km between quadrats). We measured biological variation for all species combined, separately for sessile and mobile species, and for the numerically dominant species. The data analyses indicated that horizontal biological variation was never higher than vertical biological variation, not even at the regional scale, providing support for our hypothesis. Overall, our findings suggest that studies comparing spatial scales of biological variation should consider the underlying environmental variation in addition to simply scale alone.

Predators can exert nonconsumptive effects (NCEs) on prey, which often take place through prey behavioural adjustments to minimise predation risk. As NCEs are widespread in nature, interest is growing to determine whether NCEs on a prey species can indirectly influence several other species simultaneously, thus leading to changes in community structure. In this study, we investigate whether a predator can exert NCEs on a foundation species and indirectly affect community structure. Through laboratory experiments, we first tested whether the predatory marine snail Acanthina monodon exerts negative NCEs on larviphagy (consumption of pelagic larvae) and phytoplankton filtration rates of the mussel Perumytilus purpuratus, an intertidal foundation species. These hypotheses stem from the notion that mussels may decrease feeding activities in the presence of predator cues to limit detection by predators. Afterwards, a field experiment tested whether the presence of A. monodon near mussel beds leads to higher colonisation rates of invertebrates that reproduce through pelagic larvae (expected under a lower larviphagy in P. purpuratus) and to a lower algal biomass on P. purpuratus shells (expected under a lower metabolite excretion in the mussels), thereby changing the community structure of the species typically found in P. purpuratus beds. The laboratory experiments revealed that waterborne cues from A. monodon limit the larviphagy and filtration rates of P. purpuratus. In turn, the field experiment showed that A. monodon cues led to greater abundances of barnacles and bivalves and a lower algal biomass in P. purpuratus beds, thus altering community structure. Overall, this study shows that a predator can indirectly affect community structure through NCEs on an invertebrate foundation species. As invertebrate foundation species are ubiquitous worldwide, understanding predator NCEs on these organisms could help to better understand community regulation in systems structured by such species.

Improving our understanding of stability across spatial scales is crucial in the current scenario of biodiversity loss. Still, most empirical studies of stability target small scales. We experimentally removed the local space‐dominant species (macroalgae, barnacles, or mussels) at eight sites spanning more than 1000 km of coastline in north‐ and south‐central Chile, and quantified the relationship between area (the number of aggregated sites) and stability in aggregate community variables (total cover) and taxonomic composition. Resistance, recovery, and invariability increased nonlinearly with area in both functional and compositional domains. Yet, the functioning of larger areas achieved a better, albeit still incomplete, recovery than composition. Compared with controls, smaller disturbed areas tended to overcompensate in terms of total cover. These effects were related to enhanced available space for recruitment (resulting from the removal of the dominant species), and to increasing beta diversity and decaying community‐level spatial synchrony (resulting from increasing area). This study provides experimental evidence for the pivotal role of spatial scale in the ability of ecosystems to resist and recover from chronic disturbances. This knowledge can inform further ecosystem restoration and conservation policies.

Beta diversity measures the spatial variation in species composition. Because it influences several community attributes, studies are increasingly investigating its drivers. Spatial environmental heterogeneity is a major determinant of beta diversity, but canopy‐forming foundation species can locally modify environmental properties. We used intertidal communities dominated by the canopy‐forming alga Mazzaella laminarioides as a model system to examine how a foundation species affects spatial environmental heterogeneity and the resulting beta diversity. Since canopies were found to reduce the spatial variation of temperature and desiccation during low tides, we hypothesized that canopies would decrease understory beta diversity, which we tested through a field experiment that contrasted canopy removal with presence treatments over 32 months. The beta diversity of sessile species was always lower under canopies, but canopies never affected the beta diversity of mobile species. The observed responses for sessile species may result from their abundance being more dependent on spatial abiotic variation than for mobile species, which can occur in stressful areas while temporarily foraging or in transit to other areas. These responses may likely apply to other systems exhibiting canopy‐forming foundation species hosting sessile and mobile species assemblages.